This invention relates generally to the diagnosis of medical shock-related conditions and to instruments for this purpose. More particularly, the invention relates to methods and apparatus for the non-invasive detection of pre-shock, shock and shock-related conditions (other related causes of cardio-pulmonary distress), and for assisting in a patient""s recovery from these conditions by monitoring changes in capillary flow in skin areas of peripheral body organs.
The normal skin color at most sites on the human body is generally pink. Skin color depends on the amount of blood flowing in the capillaries through which blood flows from the arterioles to the venules. The present invention resides in non-invasive detection of hemodynamic changes in the skin arteriolar-capillary flow during states of pre-shock, shock and cardio-pulmonary distress. These changes are indicative of a reduction in blood delivery to an organ of the body.
Expressed in its simplest terms, shock is the consequence of an inadequate delivery of blood to a major organ of the human body. Unless shock is promptly treated, this deprivation of blood may give rise to a disturbance in the metabolism of the organ with a resultant damage thereto. Because of the serious consequences of shock, its detection and treatment is regarded medically as an emergency procedure in which time is of the essence.
Cellular damage to an organ may be reversed by prompt treatment of shock. But it is otherwise irreversible and may lead to the death of the patient. Recovery from shock therefore depends on the promptness of treatment. However, before a patient can be treated for shock he must first be diagnosed to determine whether the patient is actually experiencing shock.
The treatment to be administered to a patient in shock depends on the nature of his condition. For example, for some shock conditions the appropriate treatment includes fluid resuscitation and the drug dopamine which acts to increase arterial perfusion pressure. Treatment for a shock condition must be administered with extreme care while the patient is being monitored.
A significant aspect of diagnostic instrumentation in accordance with the invention is that it is adapted to monitor as well as to detect shock-related conditions in a non-invasive manner. Using this instrumentation, one can make, even in a pre-hospital setting, an early diagnosis of shock as well as determine whether the drug being administered to a patient in shock is having the desired therapeutic effect.
Medical authorities classify shock syndrome in the follow five categories:
(1) Hypovolemic shock
(2) Septic shock
(3) Cardiogenic shock
(4) Obstruction to cardiac filling shock
(5) Neurogenic shock
Hypovolemic shock, the most common type of shock, is caused by a massive loss of blood, plasma or fluid from the body of a patient, or the loss of fluid from an intravascular compartment. Such losses may be due to dehydration, vomiting, diarrhea, burns, or because of the abusive use of diuretics. A loss of blood and plasma is experienced in hemorrhagic shock such as in cases of blunt and penetrating trauma injuries, gastrointestinal bleeding, or Gynecologic/Obstetric bleeding. Many cases of bleeding are occult (e.g. slow internal bleeding), and therefore can not be diagnosed early.
Septic shock is caused by bacterial infection in which an endotoxin is released into the blood stream. The sequestration and pooling of blood in various vascular compartments reduces the availability of blood for the perfusion of other vital organs.
Cardiogenic shock is usually attributed to a massive myocardial infarction caused by extensive damage to the myocardium. This may be the result of arrhythmia in a patient suffering from heart disease. In this category of shock syndrome, the heart fails to pump properly, with a consequent reduction in arterial blood.
Obstruction to cardiac filling shock takes place when this filling activity is lessened or arrested by a massive pulmonary embolism, or by space-occupying lesions. Neurogenic shock results from a severe spinal cord injury, or from a massive intake of a depressant drug, causing a loss of vasometric tone.
The five categories of shock syndrome each represent other causes of cardio-pulmonary distress, or a shock-related condition. The term xe2x80x9cshock-related conditionxe2x80x9d, as used hereinafter, is ended to embrace all five categories.
The onset of a shock condition is characterized by the reduction in blood flow to skin tissue (decreased skin perfusion). This reduction in skin perfusion is the result of a profound vasoconstriction of the skin tissue arterioles, which leads to decreased capillary flow, and a resultant poor perfusion to the skin. In order to diagnose an early stage of shock, one must detect this early reduction in skin capillary flow. A useful clinical, bed-side test for poor skin perfusion is an estimation of Capillary Filling Time (CFT). When applying pressure onto a specific skin area, the capillaries below the depressed area collapse and blood is blanched therefrom, hereby causing the skin color in the depressed skin area to whiten. Upon abrupt release of this pressure, blood flows back into the capillaries and the original skin color is recovered.
CFT is defined as the time it takes for the original pink skin color to return after it had been blanched. In clinical practice, prolongation of the CFT for more than 2 second is considered a state of shock resulting from poor skin perfusion. This well-known bed-side test, although subjective and inaccurate, is an important vital sign of a shock state. If an appropriate treatment has not been given early enough, the shock condition will continue to deteriorate, the arteriolar vasoconstriction will increase even further, as reflected by prolongation of the CFT, blood pressure will fall, and the patient may die. However, an appropriate prompt treatment at the early stage of shock will decrease vasoconstriction and shorten the CFT.
Known non-invasive methods to diagnose shock do not evaluate perfusion. These methods rely on the following cardiovascular parameters:
Blood pressure. An indirect parameter of shock. The measurement of blood pressure identifies shock only in its late stages when blood pressure drops (uncompensated shock).
Heart rate. An indirect parameter of shock. The specificity of this measurement is low because heart rate is also increased by other common physiological conditions, such as anxiety and pain.
The advantage gained by measuring the rate of blood perfusion by way of CFT instrumentation is that it enables early detection of a shock syndrome (compensated shock, prior to the reduction of blood pressure) and indicates its severity. This makes possible prompt treatment of patients who can then survive a shock-related condition which may be fatal if untreated or if treated too late.
Disclosed in U.S. Pat. No. 3,698,382 is an apparatus for measuring veno filling time which applies intermittent and uniform pressure to the skin of a patient. This instrument which measures capillary flow changes secondary to the compression of a vein comprises a light source or illuminating a skin area and photoelectric monitoring means sensitive to the coloration of the skin area. The instrument measures the rate at which color returns to the skin area after pressure thereon is released. However, there are major differences between the ""382 apparatus and apparatus in accordance with the invention in that the former measures capillary flow changes resulting from mechanical pressure applied to a nearby vein and these changes in flow do not reflect a state of shock.
When measuring CFT it is essential that pressure be applied only to capillary vessels while maintaining normal blood flow. In a preferred embodiment of an apparatus in accordance with the invention, a programmable mechanical unit applies an accurate measurable amount of pressure to the skin.
In order to diagnose the condition of shock, one must detect capillary flow changes resulting from the physiologic stress of shock. These changes in capillary flow are due to vasoconstriction and are not related to mechanical pressure applied to a nearby vein. When measuring CFT, it is vital that pressure be applied only to the capillary vessels while maintaining normal venous flow. In contradistinction to the apparatus in the ""382 patent, an apparatus in accordance with the invention uses a programmable mechanical unit that applies accurate measurable pressure to the skin, which increases gradually, until a point of maximal skin whitening has been detected. This technique makes it possible to find the MINIMAL blanching pressure which results in maximal whitening. At minimal blanching pressure, blood is moved away from the capillaries while maintaining normal flow in the veins. This technique is the hallmark of measuring true systemic changes in capillary flow.
The ""382 patent apparatus is subject to interference from external light sources and therefore requires an opaque housing for the monitoring apparatus. The apparatus does not measure skin temperature which has an independent effect on capillary flow. In addition, the mechanical arrangement required for maintaining uniform pressure in order to attain more accurate readings is cumbersome and costly.
They are also relatively complex and expensive and difficult to interpret clinically (laser Doppler devices for example). Time is of the essence in the diagnosis and treatment of shock, yet known types of skin capillary flow instrumentation are incapable of facilitating rapid diagnosis and treatment of shock. It is vital that skin capillary flow instruments have a high order of accuracy so that their readings indicate the severity of the shock or shock-related condition.
Studies published in the medical literature over the last two years demonstrate that ski temperature independently influences the skin capillary flow. One major limitation of prior skin capillary flow measurement devices is that they do not take into account skin temperature, and therefore do not correlate the measurement to skin temperature. This correlation enables real-time analysis of the state of shock. In contradistinction, a device in accordance with the invention measures skin temperature prior to each CFT measurement so that every CFT measurement is correlated to the change in skin temperature.
Of general background interest is U.S. Pat. No. 4,494,550 which discloses apparatus for the non-invasive detection of venous and arterial blood flow drainage disorders which is designed for the detection of flow abnormalities in the large vessels of a limb. Also of background interest is U.S. Pat. No. 5,050,613 (1991) which discloses a vascular testing apparatus. This includes capillary blood flow sensors to measure the blood flow of a patient. This diagnostic tool acts to determine the overall vascular integrity of a patent, but is unable and does not diagnose shock or shock-related conditions
In view of the foregoing, the main object of this invention is to provide a diagnostic method and an instrument for carrying out the method to determine accurately whether a patient is suffering from a state of shock and shock-related conditions, as well as to measure and monitor the severity of this physiologic condition.
In particular, an object of this invention is to provide a non-invasive method and apparatus adapted to detect pre-shock, shock and shock-related conditions by ongoing measurements of the patient""s capillary filling time (CFT).
A significant advantage of an apparatus in accordance with the invention is that it can expedite recovery by monitoring changes in capillary flow in skin areas of peripheral body organs. The CFT measuring instrument provides a rapid yet accurate reading of the patient""s condition, making it possible to treat the patient without delay to avoid damaging consequences.
It is also an object of this invention to provide a CFT diagnostic instrument which is of relatively simple design and easy to operate.
Briefly stated, these objects are attained in a diagnostic medical instrument adapted to determine whether a patient is suffering from a pre-shock, shock, or shock-related condition. Some shock-related conditions are related to inadequate flow in a specific organ. These medical conditions are common in patients after orthopedic surgery, flap reconstruction surgery, or patients who suffer from a severe peripheral vascular disease. By being highly sensitive to changes in capillary flow, an apparatus in accordance with the invention is applicable to these medical shock-related conditions.
The instrument is used in a capillary filling time test procedure in which a skin area in the patient overlying blood-filled capillaries which normally display a pink color, is depressed to expel blood from the capillaries and to blanch the skin and impart a white color thereto. When a point of blanching has been attained at a minimal pressure point, the pressure is then released to permit blood to flow back into the capillaries and cause the skin to regain its natural pink color. Using this minimal blanching pressure technique, blood is withdrawn from the capillaries whereas venous blood flow remains almost intact.
The instrument includes a color sensor trained on the skin area and responsive to light reflected therefrom to produce a first signal at the point in time the depressed skin color is blanched from pink to white and pressure is released when blanching at minimal pressure is attained, to later produce a second signal at the point in time at which the skin color regains its natural pink color. When the post-blanching skin color corresponds to a pre-test natural color, the CFT can be detected by recording the time which has elapsed from the maximal blanching point to this final point. In other words, the time elapsing between the first signal (starting point of minimal blanching pressure release) and the second signal (final point where post-blanching color equals pre-test color) is measured to provide a CFT index indicative of the patient""s condition at the time the test was conducted.
For each pre-determined time interval, this measurement is repeated and a new CFT is recorded.
The device will continue measuring CFT every 30 seconds to 1-5 minutes (this depends on clinical demands), and a change of CFT over time will be recorded and monitored. This change in CFT, or d[CFT]/d[t], reflects skin perfusion changes over time and measures deterioration or improvement of shock state.
In one preferred embodiment of the invention, the color sensor includes a video camera trained on the skin area of the patient and responsive to light reflected from this area to yield an image signal whose character depends on the existing color of the skin.
In another embodiment, the skin area is illuminated by a beam of light modulated at a predetermined frequency, the pulsed light reflected from this area being intercepted by a photosensor whose output signal is indicative of the skin color.